Polyphase electric energy meter

ABSTRACT

A polyphase electric energy meter comprises a microcontroller with a front end that converts analog current input signals and analog voltage input signals to digital current and voltage samples for processing by the microcontroller. The front end includes separate input channels, each for one of the current input signals with a sigma-delta modulator followed by a decimation filter. The front end further includes a common input channel for all voltage input signals with a multiplexer, an analog-to-digital converter and a de-multiplexer. The separate input channels and the common input channel provide the digital current and voltage samples for processing by the microcontroller.

This application is a divisional of application Ser. No. 11/963,546,filed Dec. 21, 2007, which claims priority to German Patent ApplicationNo. DE 10 2007 001 221.9, filed Jan. 5, 2007, the entiretiesincorporated herein by reference.

The invention relates to a polyphase electric energy meter.

BACKGROUND

In a typical three-phase electric energy meter current input signals arederived from the three phases with current transformers and voltageinput signals are derived from the three phases with a resistive voltagedivider. The current and voltage input signals are sampled and thecurrent samples are multiplied with the voltage samples to obtainelectric energy samples which are cumulated to provide an indicationrepresentative of consumed electric energy.

In an advanced electric energy meter the current and voltage inputsignals are converted to digital input samples for further processing bya microcontroller. One straight-forward approach is to use separateinput channels, each for one of the three current or voltage inputsignals and each with an analog-to-digital converter (ADC). In this“synchronous” approach all input signals are processed in parallel andsynchronously. With high accuracy requirements over a large dynamicrange, e.g. smaller than 1% over a range of 1:2000, high resolution (atleast 16-bit) ADCs are needed that are usually implemented with asigma-delta modulator followed by a decimation filter. While theapproach promises to be successful, it requires a large die space and isexpensive. An alternative approach is to use a single high resolutionADC with an input multiplexer and an output de-multiplexer. In this“sequential” approach the current and voltage input signals aresequentially switched to the input of the ADC and the resulting digitalsamples are corrected in phase to compensate for the delays introducedby the sequential sampling. The sequential approach needs less diespace, but requires a complex analog-to-digital converter to combine thehigh resolution requirements with the need to multiplex through allcurrent and voltage signals.

SUMMARY

The invention provides a polyphase electric energy meter comprising amicrocontroller with a front end that offers high resolution at moderatedie space requirements.

Specifically, the polyphase electric energy meter of the inventioncomprises a microcontroller with a front end that converts analogcurrent input signals and analog voltage input signals to digitalcurrent and voltage samples for processing by the microcontroller. Thefront end includes separate input channels, each for one of the currentinput signals with a high resolution analog-to-digital converter,preferably a sigma-delta modulator followed by a decimation filter. Thefront end further includes a common input channel for all voltage inputsignals with a multiplexer, an analog-to-digital converter and ade-multiplexer. The separate input channels and the common input channelprovide the digital current and voltage samples for processing by themicrocontroller.

The invention is based on the understanding that only the current inputsignals, due to their possibly high dynamic range, need ananalog-to-digital conversion at a high resolution and that the voltageinput signals with their small dynamic range can be sampled sequentiallyat a moderate resolution. Thus, for a three-phase meter, only three highresolution are needed ADCs for the three current input signals and asingle ADC of a moderate resolution for the voltage input signals.

In a preferred embodiment, the multiplexer has one input for eachvoltage input signal and at least one additional input for an auxiliaryinput signal such as a temperature signal or a battery voltage signal.Since the voltage samples are taken at a moderate rate, the multiplexercan be implemented with additional time slots so that more than just thevoltage input signals can be processed in the single common inputchannel.

When an application requires the neutral current to be measured inaddition to the three live currents, the front end comprises three ofthe separate input channels, each for one of three current phases, andan additional separate input channel for the neutral current inputsignal. Alternatively the multiplexed ADC can be used if a reduction ofaccuracy for the conversion of the neutral current is acceptable.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and features of the invention will become apparentfrom the following detailed description with reference to the appendeddrawings, wherein:

FIG. 1 is a schematic block diagram of a microcontroller incorporating afront-end; and

FIG. 2 is a block diagram of a polyphase electric energy meterimplemented with a microcontroller as illustrated in FIG. 1.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The microcontroller 10 generally shown in FIG. 1 includes a front-end 12which has a plurality of analog inputs and a plurality of digitaloutputs. The analog inputs are adapted to receive external analog inputsignals and the digital outputs are internal to the microcontroller forfurther processing therein.

With reference to FIG. 2, the front-end 12 includes three parallel inputchannels, each with a fully differential programmable gain amplifier PGAand a sigma-delta modulator followed by a decimation low-pass filter.The differential inputs of the amplifier PGA receive analog currentinput signals I_(A), I_(B) and I_(C), respectively, and the decimationfilters provide corresponding digital current output samples I_(AD),I_(BD) and I_(CD), respectively.

The front-end 12 further includes a multiplexer MUX with four analoginputs receiving voltage input signals V_(A), V_(B), V_(C) and V_(N),respectively, and a number of further optional inputs for application ofauxiliary external or internal signals. The output of multiplexer MUX isconnected to the input of an analog-to-digital converter ADC, thedigital output of which is connected to a de-multiplexer De-MUX. thede-multiplexer De-MUX provides digital voltage samples V_(AD), V_(BD),V_(CD), and V_(ND), respectively, and one or more optional parametersamples P.

A block “Synchronization” in the front-end 12 synchronizes operation ofall sigma-delta modulators, of the multiplexer MUX and de-multiplexerDe-MUX, and of the ADC.

The electric energy meter is connected to the three phases A, B and C ofa three-phase power source 14 which feeds a three-phase load 16.Specifically, each phase has an associated current transformer CT_(A),CT_(B) and CT_(C), respectively, and a resistive voltage divider VD_(A),VD_(B) and VD_(C), respectively. In a well-known manner, the currenttransformers CT_(A), CT_(B) and CT_(C) generate the current inputsignals I_(A), I_(B) and I_(C), and the voltage dividers VD_(A), VD_(B)and VD_(C) provide the voltage input signals V_(A), V_(B), V_(C). Theneutral voltage signal V_(N) is applied directly to a correspondinginput of the multiplexer MUX. Optional input signals such as externalparameters (temperature, battery voltage, etc.) or internal analogsignals are applied to further inputs of multiplexer MUX.

In operation, the analog current and voltage input signals are convertedto digital samples for further processing by the microcontroller.Specifically, the current input signals are analog-to-digital convertedat a high resolution (e.g., at least 16-bit resolution) in parallel andsynchronously by the three separate input channels, each with aprogrammable gain amplifier and a sigma-delta modulator followed by adecimation filter. Thus, three digital current samples I_(AD), I_(BD)and I_(CD) are available at the output of the front-end 12. In turn, thevoltage input signals are sequentially applied to the ADC, which mayhave a moderate resolution of, e.g., 12 bits, and corresponding digitalvoltage samples V_(AD), V_(BD) and V_(CD) are available at the outputsof the de-multiplexer De-MUX.

The digital current and voltage samples at the output of the front-end12 are further processed by the microcontroller by means of softwarewhich accounts for the delays of the multiplexed voltage samples.

In some applications, an additional input channel is included with anassociated further current transformer placed in the neutral line. Thisadditional input channel may be similar to the three separate inputchannels and include a programmable gain amplifier followed by asigma-delta modulator and a low-pass decimation filter.

Those skilled in the art to which the invention relates will appreciatethat the described embodiments are merely representative embodiments andthat there are variations of the described embodiments and otherembodiments within the scope of the claimed invention.

1-6. (canceled)
 7. A polyphase analog front end comprising: a pluralityof programmable gain amplifiers (PGA), wherein each PGA is adapted toreceive a current signal; a multiplexer that is adapted to receive aplurality of voltage signals; a plurality of sigma-delta modulators,wherein each sigma-delta modulator is coupled to at least one of thePGAs; an analog-to-digital converter (ADC) that is coupled to themultiplexer; and a synchronizer that is coupled to each of thesigma-delta modulators and to the ADC.
 8. The polyphase analog front endof claim 7, wherein the polyphase analog front end further comprises adecimation filter that is coupled to each sigma-delta modulator.
 9. Thepolyphase analog front end of claim 7, wherein the polyphase analogfront end further comprises a demultiplexer that is coupled to the ADC.10. A system comprising: a source having a plurality of phases; a loadthat is coupled to each phase of the source; a plurality of currenttransformers, wherein each transformer is coupled to at least one of thephases; a plurality of voltage dividers, wherein each voltage divider iscoupled to at least one of the phases; a microcontroller having ananalog front end, wherein the analog front end includes: a plurality ofprogrammable gain amplifiers (PGA), wherein each PGA is coupled to atleast one of the transformers; a multiplexer that is coupled to each ofthe voltage dividers; a plurality of sigma-delta modulators, whereineach sigma-delta modulator is coupled to at least one of the PGAs; ananalog-to-digital converter (ADC) that is coupled to the multiplexer;and a synchronizer that is coupled to each of the sigma-delta modulatorsand to the ADC.
 11. The system of claim 10, wherein the system furthercomprises a decimation filter that is coupled to each sigma-deltamodulator.
 12. The system of claim 10, wherein the system furthercomprises a demultiplexer that is coupled to the ADC.